U.S. Pat. No. 5,129,267, entitled "Flow Line Sampler", describes a sampling device for obtaining measurements of product in a flow line. A sample tube is attached to an aperture in the flow line. A piston moves reciprocally within the sampling tube, thereby removing a sample of the product from the flow line and subsequently replacing it. While the sample is in the sample tube, a non-intrusive sensor, such as a nuclear magnetic resonance sensor, measures physical properties of the sample. One feature of the sampling device is that its piston may have a hollow cavity into which a material of known physical properties may be placed, and measured for calibration purposes.